1. Field of Invention
The present invention relates to image information processing techniques in radiography, and more particularly to a method for enhancing image information in radiography.
2. Description of Prior Art
Radiographic technique is now widely used in industrial Nondestructive Detections (NDTs), medical diagnoses and science researches. The basic principle of radiography is that different absorptivities of different materials to X rays are used for forming different gradation values of pixels in a scanned image acquired by X rays penetrated an object. The energy of rays is exponentially attenuated when the thickness of the object to be scanned increases. Therefore, the difference among gradation values for border contours of a scanned image of a thicker object is much less than that of a thinner one, that is to say, the contours inside the object will be more blurred with increasing of the thickness of the object to be scanned. In the scanned image, it is represented by low sharpness of the difference in gradation values for a dark region. Furthermore, human's eyes have lower resolving capability for contours in a dark region than that for contours in a bright region. Thus, the image for objects in the dark region is not clear. Therefore, there is a need for enhancing the contrast of the scanned images.
Presently, processes for enhancing image information mainly include gradation transformation, histogram equalization, border enhancement and the like. The gradation transformation method and the histogram equalization method both use the dot operation rules. So-called dot operation refers to mapping an input image into an output image wherein each pixel dot in the output image is only determined by the value of the corresponding input pixel dot. The dot operation is generally used for changing the gradation range and distribution, and is an important tool for image digitalization and image display. During enhancing the contrast of the scanned images, conventional dot operation does not consider the influence on a certain pixel by its neighborhood, but simply maps one gradation value onto another gradation value. Such a procedure impairs one gradation range while enhancing another gradation range, typically, such as logarithm transformation and exponent transformation. An edge enhancement method is also referred to as sharpening, and the object thereof is to clarify edges of images in order to recognize and process a target. The existing methods include gradient method, Laplacian operator and high-pass filtering etc. The final object of these processing methods is to make images clearer, and to make details apparent. When enhancing the contrast of scanned images, the edge enhancement method directly detects edges of an image, and enhances the contrast of the detected edges, in which a ratio of enhancement is generally constant. On one hand, such a processing method has to enhance the noises in images. On the other hand, the enhancement to weak edges in the dark regions is relatively small, which can not satisfy the requirements of relatively more enhancement to the contrast of the dark regions in the scanned images.